1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a disk with laminated perpendicular magnetic recording layers.
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording system is one that uses a “dual-layer” medium. This type of system is shown in FIG. 1 with a single write pole type of recording head. The dual-layer medium includes a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL).
One type of material for the RL is a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL can be achieved by the addition of oxides, including oxides of Si, Ta, Ti, Nb and B. These oxides tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy form nonmagnetic intergranular material.
The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field H acting on the recording layer RL. The disk also includes the hard disk substrate, a seed or onset layer (OL) for growth of the SUL, an exchange break layer (EBL) to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and to facilitate epitaxial growth of the RL, and a protective overcoat (OC). As shown in FIG. 2, the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole (FIG. 1) which is the real write head (RWH) above the disk, and a secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the EBL and produces a magnetic image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field H inside the RL.
Magnetic recording media that have RLs formed of granular ferromagnetic cobalt alloys, such as the cobalt alloys used for perpendicular recording, exhibit increasing intrinsic media noise with increasing linear recording density. Media noise arises from irregularities in the recorded magnetic transitions and results in random shifts of the readback signal peaks. High media noise leads to high bit error rates. Thus to obtain higher areal densities in magnetic recording disk drives, it is necessary to decrease the intrinsic media noise, i.e., increase the signal-to-noise ratio (SNR), of the recording media. The media SNR is to first order proportional to 201 og (N1/2), where N is the number of magnetic grains per unit area in the media. Accordingly, increases in SNR can be accomplished by increasing N.
Improved media SNR can be achieved with “laminated” media. In laminated media, the single magnetic layer is replaced with a laminate of two or more separate magnetic layers that are spaced apart and magnetically decoupled by nonmagnetic spacer layers. This discovery was made for horizontal or longitudinal magnetic recording media by S. E. Lambert, et al., “Reduction of Media Noise in Thin Film Metal Media by Lamination”, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706-2709, and patented in U.S. Pat. No. 5,051,288. A laminated media structure with two perpendicular RLs separated by a nonmagnetic spacer layer (SL) is shown schematically in FIG. 3. The presence of two decoupled RLs doubles N, the number of grains per area, leading to the SNR improvement. Laminated media do exhibit improved bit error rate (BER), as expected based on the SNR increase, but they require a higher field from the write head than for media with single-layer RLs to achieve the improved BER.
The need for higher write fields is not limited to laminated media. Higher write fields are required for perpendicular media with single-layer RLs than for comparable horizontal media with single-layer RLs. This can be appreciated by reference to FIG. 2 which shows the write field H inside the RL being oriented nearly parallel to the surface normal, i.e., along the perpendicular easy axis of the RL grains, as shown by typical grain 1 with easy axis 2. The parallel alignment of the write field H and the RL easy axis has the disadvantage that relatively high write fields are necessary to reverse the magnetization because minimal torque is exerted onto the grain magnetization. For these reasons, a composite medium consisting of two ferromagnetically exchange-coupled magnetic layers has been proposed. Magnetic simulation of this composite medium shows that in the presence of a write field the magnetization of the upper magnetic layer will rotate first and assist in the reversal of the magnetization of the lower magnetic layer. This behavior, sometimes called the “exchange-spring” behavior, and various types of composite media are described by R. H. Victora et al., “Composite Media for Perpendicular Magnetic Recording”, IEEE Transactions on Magnetics, 41 (2), 537-542, Feb. 2005; and J. P. Wang et al., “Composite media (dynamic tilted media) for magnetic recording”, Appl. Phys. Lett. 86 (14) Art. No. 142504, Apr. 4 2005. Pending application Ser. No. 11/231,516 filed Sep. 21, 2005, and assigned to the same assignee as this application, describes a perpendicular magnetic recording medium with an exchange-spring structure.
What is needed is a perpendicular magnetic recording medium that has high recording density and high SNR and yet is easy to write.